A. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium for recording and reproduction of information, in particular to a perpendicular magnetic recording medium installed in a hard disk drive (HDD).
B. Description of the Related Art
In recent years, the demand for high density recording in magnetic recording media is increasing. The magnetic recording media at present employ a longitudinal magnetic recording system in which a magnetic recording layer of a CoCr alloy or the like is provided through an underlayer of chromium, a chromium alloy or the like on a substrate, and the recording magnetization is aligned in the direction of the substrate surface. Recently, a perpendicular magnetic recording system has gained attention in which recording is performed with the axis of easy magnetization in the recording layer being aligned in the vertical direction. In either recording system, high density recording needs high thermal stability, low noise, and reduction of magnetic spacing. As a way of reducing the magnetic spacing, a protective layer formed on the magnetic recording layer is made as thin as possible. The decrease of protective layer thickness, however, causes a reliability problem due to increased elution of cobalt from the magnetic recording layer and significant deterioration of slidability. Since a soft magnetic backing layer is necessary in the perpendicular magnetic recording system, the cobalt elution becomes a more severe problem in a soft magnetic backing layer made of a cobalt-based alloy.
A means to avoid the cobalt elution caused by a thin protective layer has been proposed in which an element of the platinum group other than platinum or an element with negative electrode potential is added to the magnetic layer. (See, for example, Japanese Unexamined Patent Application Publication No. 2003-223707.) The technique of adding an element to the magnetic film is barely effective for small additions, while degradation of magnetic and electromagnetic conversion performances results from large additions of the element. The elements in the platinum group other than platinum have a large atomic radius, requiring adjustment of the composition and deposition process of the intermediate layer and the underlayer.
Another method has been proposed in which an intermediate layer after passivation treatment is provided between the magnetic layer and the protective layer. (See, for example, Japanese Unexamined Patent Application Publication No. H8-173155.) The provision of an intermediate layer, however, raises a problem of increased magnetic spacing due to the addition of this layer. Still another technique has been proposed in which chromium and hard micro particles are added in the protective layer. (See, for example, Japanese Unexamined Patent Application Publication No. 2002-100036.) However, the protective layer must perform other functions, including ensuring slidability in addition to the prevention of cobalt elution. To limit the material of the protective layer to avoid cobalt elution imposes an unfavorable restraint in achieving the best performance in other functions.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.